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    <title>Document</title>
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    <script>
      const flat = (arr) => {
        return arr.reduce((prev, curr) => {
          return prev.concat(Array.isArray(curr) ? flat(curr) : curr);
        }, []);
      };
      const arr = [
        1,
        2,
        3,
        4,
        [1, 2, 3, [1, 2, 3, [1, 2, 3]]],
        5,
        "string",
        { name: "hjj" },
      ];
      console.log(flat(arr)); // [1, 2, 3, 4, 1, 2, 3, 1, 2, 3, 1, 2, 3, 5, 'string', { name: "hjj" }]
    </script>
  </body>
</html>
